In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. Ion implantation systems are often utilized to dope a workpiece, such as a semiconductor wafer, with ions from an ion beam, in order to either produce n- or p-type material doping, or to form passivation layers during fabrication of an integrated circuit. Such beam treatment is often used to selectively implant the wafers with impurities of a specified dopant material, at a predetermined energy level, and in controlled concentration, to produce a semiconductor material during fabrication of an integrated circuit. When used for doping semiconductor wafers, the ion implantation system injects a selected ion species into the workpiece to produce the desired extrinsic material.
A typical ion implanter includes an ion source, an ion extraction device, a mass analysis device, a beam transport device and a wafer processing device. The ion source generates ions of desired atomic or molecular dopant species. These ions are extracted from the source by an extraction system, typically a set of electrodes, which energize and direct the flow of ions from the source, forming an ion beam. Desired ions are separated from the ion beam in a mass analysis device, typically a magnetic dipole performing mass dispersion or separation of the extracted ion beam. The beam transport device, typically a vacuum system containing a series of focusing devices, transports the ion beam to the wafer processing device while maintaining desired properties of the ion beam. Finally, semiconductor wafers are transferred in to and out of the wafer processing device via a wafer handling system, which may include one or more robotic arms, for placing a wafer to be treated in front of the ion beam and removing treated wafers from the ion implanter.
Ion sources (commonly referred to as arc ion sources) generate ion beams used in implanters and can include heated filament cathodes for creating ions that are shaped into an appropriate ion beam for wafer treatment. U.S. Pat. No. 5,497,006 to Sferlazzo et al., for example, discloses an ion source having a cathode supported by a base and positioned with respect to a gas confinement chamber for ejecting ionizing electrons into the gas confinement chamber. The cathode of the Sferlazzo et al. is a tubular conductive body having an endcap that partially extends into the gas confinement chamber. A filament is supported within the tubular body and emits electrons that heat the endcap through electron bombardment, thereby thermionically emitting ionizing electrons into the gas confinement chamber.
Carbon has emerged as a widely used dopant in the semiconductor industry for a wide variety of material modification applications. For example, carbon implantation is often used to inhibit diffusion of co-dopants or for enhancing stability of the doped region. In this regard, carbon dioxide and/or carbon monoxide are two commonly used dopant gas sources for carbon implantation. The residual oxygen from the disassociation of the carbon molecule can oxidize the chamber liners as well as damage the cathode shield causing a premature failure of the ion source. Moreover, residual carbon deposits and flaking due to the cracking of carbon dioxide and/or carbon monoxide are also known to shorten ion source lifetimes.